1. Field of the Invention
The present invention relates to a source driver, and more particularly to a source driver for regulating the slew rate of the output signals.
2. Description of Related Art
A source driver plays an important role in a liquid crystal display (LCD) panel, which converts digital video data into a plurality of voltage signals and delivers the voltage signals to pixels on a display panel for displaying a frame. The source driver includes a plurality of output buffers for enhancing the voltage signals. Due to different characteristics of the electronic elements, and the process variation, the voltage signals outputted from these output buffers may not have identical slew rate for driving pixels on the display panel. As a result, the time for orienting liquid crystals corresponding to locations of the pixels is different, and the representation of the displayed image is uneven.
Generally, in the driving system of the LCD, the polarity of the voltage signal delivered to a certain pixel must be periodically converted for avoiding a residual image phenomenon caused by liquid crystal polarization. There are three types of polarity inversion for driving the display panel, i.e. frame inversion, column inversion, and dot inversion. Taking the dot inversion as an example, the adjacent pixels in one frame are driven by the driving voltages with opposite polarities, and the pixels in the same location of two continuous frames are also driven by the driving voltages with opposite polarities. Usually, the driving voltage with positive polarity is greater than a common voltage coupled to the liquid crystal, and the driving voltage with negative polarity is smaller than the common voltage.
Since the driving voltages with opposite polarities have different voltage levels, the output buffers respectively used for enhancing the driving voltages with different polarities may be composed of different electronic elements. For example, the output buffer suitable for enhancing the driving voltage with positive polarity may include an N-type differential pair for receiving driving voltage with high level and controlling the operation of the output buffer. Contrarily, the output buffer suitable for enhancing the driving voltage with positive polarity may include a P-type differential pair for receiving driving voltage with low level and controlling the operation of the output buffer. As the foregoing description, due to different characteristics of the electronic elements, the voltage signals outputted from the output buffers are likely to have different slew rates.
Obviously, the liquid crystals driven by the voltage signals are oriented in different time due to the different slew rates of the voltage signals. As a result, gray scales of the displayed image are displayed unevenly, and the flickers perceived by human eyes are more serious when performing the polarity inversion. The display quality will be degraded. It is desirable to design a proper source driver to solve the said problem.